Motion Chapter 2

Section 1: Describing Motion What you will learn: Distinguish between distance and displacement. Explain the difference between speed and velocity Interpret motion graphs.

Motion Motion occurs when an object changes it’s position relative to a reference point. A reference point is used to create a frame of reference, which is a coordinate system where the position of the object(s) is/are measured.

Motion Continued… Distance is how far an object has moved. SI unit of distance is the meter (m). Displacement is the distance and direction of an object’s change in position from the starting point.

Speed Speed is the distance an object travels per unit of time. Calculating Speed: Any change over time is called a rate. Speed = Distance (m) Time (s) Ex: Acura NSX top speed: 168 mph

Solve This Speed Problem: A car travels at a constant speed and covers a distance of 750m in 25s. What is the car’s speed? 1. Write what you know: d= 750m t= 25s 2. The variable you need is speed 3. s= d/t 4. Use numbers: s= 750/25= 30m/s

More Speed Problems 1. An elevator travels from the first floor to the 60th floor, a distance of 210m, in 35s. What is the elevators speed? s= d/t s= 210/35 = 6m/s 2. How far does a car travel in 0.75h if it is moving at a constant speed of 88km/h? d= st d= 88km/h)(.75h) = 66km

Changing Speed Average speed describes speed of motion when the speed is changing. Average speed is the total distance traveled divided by the total time of travel.

Instantaneous Speed Instantaneous speed is the speed at a given point in time. In this example, the instantaneous speed is 100mi/h. This person needs to slow down!!! Two examples of motion in which the instantaneous speed changes are acceleration and deceleration.

Graphing Motion The motion of an object over a period of time can be shown with a distance-time graph. Time is plotted on the x-axis and distance on the y-axis.

Velocity Velocity is a term that describes the speed of an object AND IT’S DIRECTION. To tell if you are in danger of a hurricane you need to know how fast it is moving and in what direction.

Velocity Continued… If the cars in this picture maintain the same speed, does their velocity stay the same or change? Why? Velocity changes because direction is constantly changing.

Did We Reach Our Learning Goals for Section One? 1. Distinguish between distance and displacement. 2. Explain the difference between speed and velocity. 3. Interpret this graph:

Section 2: Acceleration What you will learn: Identify how acceleration, time, and velocity are related. Explain how positive and negative acceleration affect motion Describe how to calculate the acceleration of an object.

Acceleration Review: Velocity is the change of position. New: Acceleration is the change of velocity. Velocity includes both the speed AND direction of an object. A change in velocity can be a change in: 1. How fast something is moving OR 2. A change in the direction it is moving. Recap: Acceleration occurs when an object changes it’s speed, it’s direction, or both!

Speeding Up and Slowing Down Positive Acceleration is when the speed of an object is increasing. Negative Acceleration is is decreasing.

Let’s Do Some Practice Is a car accelerating if it is moving in a straight line at a constant speed? NO! Is a car accelerating if it is moving in a straight line but slowing down? YES! Is a car accelerating if it is moving in a circular racetrack at a constant speed?

Changing Direction Any time a moving object changes direction, it’s velocity changes and it is accelerating.

Calculating Acceleration Acceleration is the rate of change in velocity. To calculate the change in velocity: Final velocity- initial velocity To calculate acceleration (in m/s2)= a = vf –vi t

Calculating Positive Acceleration Example: An airplane starts at rest at the end of a runway and reaches a speed of 80m/s in 20s. Calculate acceleration. a = vf –vi t a = (80m/s – 0m/s) = 4m/s2 20s

Calculating Negative Acceleration Example: A skateboarder is moving in a straight line at a constant speed of 3m/s and comes to a stop in 2 s. Calculate the acceleration: a = vf –vi t a = (0m/s -3m/s) = -1.5m/s2 2 s

Did We Reach Our Learning Goals For Section 2? 1. Identify how acceleration, time, and velocity are related. 2. Explain how positive and negative acceleration affect motion. 3. Describe how to calculate the acceleration of an object.